Science | New test for Down’s syndrome unveiled

A breakthrough test for individuals with a high risk of having a child with Down’s syndrome is due to go on trial in the NHS.

Down’s syndrome is the most common chromosomal genetic disorder and is caused, in the vast majority of cases, by having three copies of chromosome 21. It is therefore known as a trisomy. We are genetically programmed to be born with 23 pairs of chromosomes and they each hold vast amounts of genetic information. Down’s syndrome is a serious life limiting condition that can have major implications for both the mother and child’s health. The news of the test has been welcomed by doctors, parents and disability groups alike.

The new test is called cell free DNA (cfDNA) and is a simple blood test. It takes a sample of blood from the mother and looks at fragments of DNA within her blood. If there is a greater amount of circulating DNA for chromosomes 13, 18 and 21 (which are the most common trisomies) then it is suggestive of that condition. The two large prospective studies to publish this research found it had a 99 per cent detection rate and a false-positive rate of only 0.1 per cent. This is far superior to traditional methods, of which have a false positive that normally lies between 3-5 per cent and would result in fewer potentially harmful invasive tests.

Sadly it does have its drawbacks. It is currently only available to higher risk mothers that have been picked up on the 11-13 week screening tests, one of which is nuchal translucency. This looks at the amount of fluid behind the neck of the fetus; an increased amount has a correlation with genetic disorders. It is also less reliable in the diagnosis of the other trisomies and these would need more invasive tests such as amniocentesis and chorionic villus sampling (CVS). The first involves collection of amniotic fluid and the second a small biopsy from the placenta. Both carry approximately a 1 in 100 risk of miscarriage.

The cfDNA test is being piloted at Great Ormond Street Hospital and, if successful, will be rolled out nationally. There are concerns about the cost of the test and it is currently planned to be used in a supplementary manner alongside current prenatal testing. It raises further debate on pre-natal testing and its uses. The test is currently available privately at 10 weeks and, with a reported 99 per cent positive detection rate, is an extremely reliable screening test. Will this raise the number of abortions carried out in light of these genetic tests or will it better prepare parents of what to expect where their child is born.